In recent years, vigorous development has been directed to power devices in which a silicon carbide semiconductor. Silicon carbide (SiC) is a semiconductor material with a high hardness, having a larger band gap than that of silicon (Si). Silicon carbide has dielectric breakdown field which is one order of magnitude higher than that of silicon. Therefore, use of silicon carbide makes it possible to produce a semiconductor device which has the same breakdown voltage as, but a smaller volume than, in the case of using silicon. Use of silicon carbide allows those constituent elements which serve as resistance components to be reduced as compared to using silicon, thereby making it possible to reduce the ON resistance of the semiconductor device and decrease its power loss. A silicon carbide semiconductor device also has an advantage of being able to operate at a higher temperature than is possible with silicon. A silicon carbide semiconductor device is used as a switching element composing a switching circuit, for example.
Some attempts at reducing losses in switching circuits have been made, which involve control of switching element operations (see, for example, Patent Document 1).
Patent Document 1 discloses a technique of, in a switching circuit composed of a half bridge circuit in which metal-oxide-semiconductor field effect transistors (MOSFETs) are used as switching elements, providing a transistor current detection means to detect a current flowing in a low-side MOSFET and a diode current detection means to detect a current flowing in the body diode of the low-side MOSFET functioning as a free-wheel diode, to thereby reduce recovery losses while suppressing the through-current.
Specifically, according to Patent Document 1, in a vertical MOSFET, a diode electrode is provided so as to achieve ohmic contact with the body region but without contact with the source region, the diode electrode being deployed in an electrically insulated state from the source electrode. Thus, by detecting a current flowing between the source electrode and the drain electrode, a current flowing through the MOSFET is detected, and by detecting a current flowing between the diode electrode and the drain electrode, a current flowing through the body diode can be detected. Among a plurality of unit cells, some unit cells have a means that detects a current between the source electrode and the drain electrode and a means that detects a current between the diode electrode and the drain electrode, which respectively function as the transistor current detection means and the diode current detection means. Patent Document 1 states that recovery losses can be reduced while suppressing the through-current, by setting a dead time so that the through-current which is detected by the transistor current detection means and the recovery current which is detected by the diode current detection means are both small.
On the other hand, Patent Document 2 discloses, in such an inverter as drives a motor, a construction including a transistor bridge circuit which is composed of a plurality of transistors and a diode bridge circuit which is composed of a plurality of diodes that are free-wheel diodes, where a first current detector is disposed across a plus line and a minus line between the transistor bridge circuit and the diode bridge circuit, and a second current detector is disposed between the transistor bridge circuit and diode bridge circuit and a DC power source. Patent Document 2 states that a driving current which flows during usual driving a reverse current which flows in a backflow operation, and a regenerative current which flows in a regeneration operation can be detected by using the first current detector and the second current detector, whereby an overcurrent occurring in each operation can be detected.